[whatwg] Proposal for separating script downloads and execution
getify at gmail.com
Wed Feb 9 07:06:13 PST 2011
> Mighty conjecture, chap. Multithreading is even possible on
> microcontrollers like the Atmel ATmega32 ? so why should a modern
> operating system running on reasonable hardware not be able to do it?
In most mobile devices I've had the exposure to developing for,
multi-threading is not possible/available to me. The usual answer, for
instance for the iPhone, is that true multi-threading will tend to cause
serious drains on limited battery life, which degrades quality of
user-experience and user satisfaction. That's the only "anecdotal" evidence
that I have for how these engines may not be completely free to multi-thread
as is being suggested.
In any case, you're still missing the point. The mobile OS's (and even the
implementation details, but this HTML spec has only a slight modest ability
to affect that. The hardware/mobile-OS vendors have dozens of different
pressures that play into what they can and cannot implement, and how. Even
if the HTML spec were to say "must process script execution in a separate
thread from the rendering engine", the feasibility of that requirement may
still be overshadowed by lots of factors completely out of the control of
the specification group.
We can continue to debate what might be nice for mobile vendors to consider,
but they aren't on this list and listening to us. Who IS on this list, and
who IS interested, are developers who have real performance problems right
now. And they are creating ever more complex hacks to get around these
problems. And the spec has an opportunity to make a small foot-print change
to give them some better options for that performance negotiation.
You're also ignoring the fact that there are several other documented
use-cases for execution-deferral that are not related to mobile (or
multi-threading) at all. That maybe the 80% use-case for this proposal, but
it's certainly not the only reason we want and need a feature like this.
> Fun fact: I use mobile versions of some web sites, because they are much
> quicker, even on the desktop. Sometimes a little minimalism can go a
> long way.
We're not particularly talking about generalized web sites as much as we are
talking about complex mobile web applications like Gmail. Even in their
minimalism, the bare minimum experience they're willing to deliver is
overloading the mobile browser and so they are resorting to crazy and
In my opinion, when we see a trend toward developers having to hack around
certain parts of the functionality that don't work the way they need it to
(for real-world use-cases), then it's a good sign that we should consider
really all that helpful for the population of applications that are most in
need of this feature.
> Counter-intuitive at first, but true: More complex code is not
> necessarly faster code. More options are more options to screw up.
We have a number of well-known and well-documented experts in the realm of
page-load optimization and script loading functionality who are behind
requests like is being discussed. If we can't trust them to do correctly
with what we give them, then the whole system is broken and moot. The fact
that some developers may misunderstand and improperly use some functionality
should not prevent us from considering its usefulness to those who clearly
know the right things to accomplish with it.
It also hasn't been shown with any degree of specificity just what the fear
is of developers "screwing up" if we give them this functionality. Right now
it's a bunch of conjecture about possible misunderstandings, something which
should be easy to deal with through proper documentation, education, and
evangelism. Why are we so afraid to let the right implementations of a
functionality flourish and bubble to the top, and drown out the wrong
implementations of functionality by those are are either ignorant or
> I'm losing track in the noise of what the fundamental disagreements
> there even are any. I think the original proposal is a very good place to
The original proposal is in fact more focused on the markup-driven use-case
than on the script-driven use-case. The original proposer, Nicholas, agreed
in an earlier message that he's really more concerned with script-driven
functionality than markup driven functionality. And I completely agree with
In fact, I'd go so far as to say that the use-case for separating script
loading from its parsing/execution phase (and thus being able to
control/trigger when that phase occurs, later) is 99% driven by the
script-loaders use-case. Script loaders by and large do not use markup
semantics to accomplish their tasks (because most of them do not use
document.write("<script...."); to load scripts)
So, if we consider the spirit of the original proposal, we should examine it
in the proper context (the vast majority use-case), which is script elements
being created from script logic rather than markup.
Given that proper context, the proposal becomes something like:
1. Give a dynamic script element a "noExecute" property (a boolean property,
defaults to false, can be set to true)
2. Give a dynamic script element an "execute()" function which executes a
script that has been "deferred" by the "noExecute" property.
The problem with *that* phrasing of the proposal (compared to the
"readyState" preoloading I'm advocating) is:
1. It asks for two new unprecedented additions to the script element
specification. The other proposal asks to take the existing spec wording and
change it from a "may" to a "must" (from suggestion to requirement).
2. It has no existing browser support. The "readyState" proposal also
suggests we take the precedent of "readyState" property and the associated
"onreadystatechange" handler, from say XHR, and apply it to script elements,
something which one browser vendor (IE) has already done, and at least one
other browser vendor has said they could/would do (Opera).
In other words, the "readyState" proposal tries to address the needs of the
use-case using existing precedent and existing spec content as much as
possible, AND it has already some browser vendor support. "noExecute" has
neither of those two things, and so has a tougher uphill battle to winning
> - Just for comparison: <script src="path.js" noexecute
Several times that code snippet has been suggested in this thread, and each
time a fundamental problem is being overlooked. The way the "onload" event
CURRENTLY works is that it doesn't fire until after a script has finished
executing. There's no way we can change the semantics of "onload" now to be
run before the script executes (like when it actually finishes loading),
even though that clearly would be the more semantic way for that event to
As stated there, `onload="this.execute()"` will never execute because of
So, we'd actually be forced to add yet another event/property to the script
element (either in markup or in script) that was called "onfinishloading" or
"onloaded" (probably confusing alongside "onload") or something like that.
And *that* is conceptually pretty similar to my proposal for utilizing the
precedent of the "readyState" event progression, which has a "loaded" event
> The link tag is meant to support a "prefetch" value for the "rel"
> attribute asking to preemptively cache the resource:
> - http://blog.whatwg.org/the-road-to-html-5-link-relations#rel-prefetch
> - http://davidwalsh.name/html5-prefetch
For <link rel=prefetch> to address the use-case, some event mechanism would
HAVE to be added to the <link> tag such that the finishing of that
"prefetching" could be detected. But then what do we do once it's finished
loading, even if we *do* add some <link> event mechanism to detect it?
.execute() would be a terrible idea for <link>, because it would essentially
morph <link> into a <script> element at that point. Not only is this
significantly more confusing to web authors for that type of behavioral
overloading, but I'd guess that all the complicated semantics around the
<script> element would then have to be duplicated into a <link> tag that is
It's tempting to suggest that you would then just add a proper <script>
element with the same URL to accomplish the "execution" of it. This suffers
a similar fate to many of the hacky workarounds that currently exist: that
it's based on the assumption that the resource was cached.
Lots of resources are loaded/executed and never cached (either on purpose,
or by fault of improper headers). So, this "cache preloading" technique,
however we try to enable it, will always be flawed by that assumption, an
assumption which if it fails, causes a very costly double-load of the
Two other problems with this <link rel=prefetch> idea:
1. The current spec wording around <link rel=prefetch> is clearly that that
its just a "hint" to the browser that it should fetch that resource at some
point in the future. The use-case calls for a decisive/determinate mechanism
(like the existing script element) whereby the author can express "I need
you (browser) to load this as soon as possible" as opposed to the semantics
of <link rel=prefetch> which are more like "I think it would be a good idea,
if you get some time, to load this resource, but only if you feel like you
should, at some undetermined time in the future."
For <link rel=prefetch> to be useful to the use-case, it would need to have
that spec wording changed to be a lot more direct about the resource needing
to be loaded with a much higher priority on the "now" of it. Of course, no
load is guaranteed to start right away, but there's an important difference
between "please load asap" and "you can load whenever you feel like it".
2. The majority use-case is, as stated above, for script loaders (in script
logic) to be able to "preload"/"prefetch" script elements. AFAIK, <link
rel=prefetch> isn't defined for script-inserted <link> elements, and it
certainly (upon testing) isn't supported well in browsers. Clearly, <link
rel=prefetch> is much more tuned to the vast minority use-case, of
Even if we did force that <link rel=prefetch> be functional from
script-driven code, it would look a WHOLE LOT like what the "readyState
preloading" proposal already looks like, except it'd be more confusing
because it'd involve two different elements instead of just one.
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